1. Field of Invention
The present invention relates to fuel cells, and more particularly to methods and materials for electrically insulating and sealing fuel cell plates.
2. Discussion
A fuel cell is a device that converts chemical energy of fuels directly to electrical energy and heat. In its simplest form, a fuel cell comprises two electrodesxe2x80x94an anode and a cathodexe2x80x94separated by an electrolyte. During operation, a gas distribution system supplies the anode and the cathode with fuel and oxidizer, respectively. Typically, fuel cells use the oxygen in the air as the oxidizer and hydrogen gas (including H2 produced by reforming hydrocarbons) as the fuel. Other viable fuels include reformulated gasoline, methanol, ethanol, and compressed natural gas, among others. The fuel undergoes oxidation at the anode, producing protons and electrons. The protons diffuse through the electrolyte to the cathode where they combine with oxygen and the electrons to produce water and heat. Because the electrolyte acts as a barrier to electron flow, the electrons travel from the anode to the cathode via an external circuit containing a motor or other electrical load that consumes power generated by the fuel cell.
Currently, there are at least five distinct fuel cell technologies, each based on a different electrolyte. One class of fuel cells, which is known as a polymer electrolyte membrane (PEM) fuel cell, appears well-suited for mobile power generation (transportation applications) because of its relatively low operating temperatures (about 60xc2x0 C. to about 100xc2x0 C.) and its quick start up. PEM fuel cells use an electrolyte composed of a solid organic polymer, which is typically a poly-perfluorosulfonic acid. Other fuel cell technologies include electrolytes comprised of solid zirconium oxide and yttrium (solid oxide fuel cells) or a solid matrix saturated with a liquid electrolyte. Liquid electrolytes include aqueous potassium hydroxide (alkaline fuel cells), phosphoric acid (phosphoric acid fuel cells), and a mixture of lithium, sodium, and/or potassium carbonates (molten carbonate fuel cells). Although phosphoric acid fuel cells (PAFC) operate at higher temperatures than PEM fuel cells (about 175xc2x0 C. to about 200xc2x0 C.), PAFCs also find use in vehicle applications because of their higher efficiency and their ability to use impure hydrogen gas as fuel.
The core of a typical PEM fuel cell is a three-layer membrane electrolyte assembly (MEA). The MEA is comprised of a sheet of the polymeric electrolyte, which is about 50xcexc (microns) to about 175xcexc thick and is sandwiched between relatively thin porous electrodes (anode and cathode). Each of the electrodes usually consists of porous carbon bonded to platinum particles, which catalyze the dissociation of hydrogen molecules to protons and electrons at the anode and the reduction of oxygen to water at the cathode. Both electrodes are porous and therefore permit gases (fuel and oxidizer) to contact the catalyst. In addition, platinum and carbon conduct electrons well so that electrons move freely throughout the electrodes.
An individual fuel cell generally includes backing layers that are placed against the outer surfaces of the anode and the cathode layers of the MEA. The backing layers allow electrons to move freely into and out of the electrode layers, and therefore are often made of electrically conductive carbon paper or carbon cloth, usually about 100xcexc to 300xcexc thick. Since the backing layers are porous, they allow fuel gas or oxidizer to uniformly diffuse into the anode and cathode layers, respectively. The backing layers also assist in water management by regulating the amount of water vapor entering the MEA with the fuel and oxidizer and by channeling liquid water produced at the cathode out of the fuel cell.
A complete fuel cell includes a pair of plates pressed against the outer surfaces of the backing layers. Besides providing mechanical support, the plates define fluid flow paths within the fuel cell, and collect current generated by oxidation and reduction of the chemical reactants. The plates are gas-impermeable and have channels or grooves formed on one or both surfaces facing the backing layers. The channels distribute fluids (gases and liquids) entering and leaving the fuel cell, including fuel, oxidizer, water, and any coolants or heat transfer liquids. As discussed below, each plate may also have one or more apertures extending through the plate that distribute fuel, oxidizer, water, coolant and any other fluids throughout a series of fuel cells. Each plate is made of an electron conducting material including graphite, aluminum or other metals, and composite materials such as graphite particles imbedded in a thermosetting or thermoplastic polymer matrix.
For most applications, individual fuel cells are connected in series or are xe2x80x9cstackedxe2x80x9d to form a fuel cell assembly. A single fuel cell typically generates an electrical potential of about one volt or less. Since most applications require much higher voltagesxe2x80x94for example, conventional electric motors normally operate at voltages ranging from about 200 V to about 300 Vxe2x80x94individual fuel cells are stacked in series to achieve the requisite voltage. To decrease the volume and mass of the fuel cell assembly, a single plate separates adjacent fuel cells in the stack. Such plates, which are known as bipolar plates, have fluid flow channels formed on both major surfacesxe2x80x94one side of the plate may carry fuel, while the other side may carry oxidizer.
Because the fluids flowing within a particular fuel cell and between adjacent fuel cells must be kept separate, conventional fuel cell assemblies employ resilient o-rings or planar inserts disposed between adjacent fuel cell plates to seal flow channels and apertures. In addition, conventional fuel cell assemblies also provide electrical insulating sheets between adjacent plates to prevent individual fuel cells from short-circuiting. Although such seals and insulators are generally satisfactory, they suffer certain disadvantages. For example, freestanding o-rings and planar inserts must be carefully aligned with channels and apertures to ensure proper sealing and insulation, which is time consuming. Because of their non-standard sizes and shapes, planar inserts used in fuel cell assemblies are typically made by injection molding, compression molding, or transfer molding, which require expensive, one-of-a-kind tooling. Furthermore, many of the resilient materials used to make o-rings and planar inserts do not have the requisite chemical resistance and low modulus to adequately seal fuels cells operating at higher temperatures or employing hydrocarbon-based heat transfer fluids and coolants.
The present invention helps overcome, or at least mitigates one or more of the problems described above.
The present invention provides a process for sealing and insulating a fuel cell assembly comprised of two or more fuel cell plates. The process includes providing a fuel cell plate having first and second surfaces and applying a coating precursor on at least the first surface of the fuel cell plate. Since the coating precursor is capable of polymerizing (curing) in response to exposure to energy, the method also includes heating the coating precursor on the fuel cell plate via radiant (infrared) heating or convective heating to initiate polymerization. Useful coating precursors include epoxy nitrile resins.
The invention also provides an insulated fuel cell plate comprised of a plate having first and second surfaces and a coating precursor applied to at least one of the first and second surfaces of the plate. The coating precursor is an epoxy nitrile resin, which comprises an epoxy resin and an acrylonitrile butadiene copolymer. The epoxy nitrile resin optionally includes a thermoplastic film-former, a polyamine cross-linking agent, and a solvent.
The present invention offers certain advantages over conventional methods and designs for insulating and sealing fuel cell plates and fuel cell assemblies. For example, unlike o-rings and molded inserts, the disclosed coating precursors can be quickly and precisely applied to fuel cell plates (e.g., by screen printing) resulting in substantial cost savings. Furthermore, in contrast to many conventional resilient materials, the cured solid coatings combine good chemical resistance with excellent mechanical properties.